ADIABATIC CHANGES OF MOIST AIR NEUHOFF 435 



partly condensed and falls as rain or is suspended in the air as 

 water, and this is the rain stage. 



This stage continues until the temperature has fallen to o° C; 

 now the precipitated water freezes while at the same time partial 

 evaporation tal£es place while the temperature remains constant. 

 As soon as all the water is frozen this isotherm of freezing, this hail 

 stage or third period, is past and the fourth or snow stage begins 

 as the lowering of temperature proceeds further. 



These processes bring about different final results according as 

 the condensed water falls away from the cooling air, either immedi- 

 ately or at some subsequent time. But for mathematical study 

 we may assume that the mass of air rising up to a certain height 

 carries with it its condensed aqueous vapor and therefore remains 

 unchanged as to its total constituents. 



If the inverse process takes place when the air is sinking, then in 

 consequence of the increase of te'mperature the condensed water 

 evaporates again and the mass of air returns gradually to its former 

 condition at its initial elevation above sea-level or its initial pres- 

 sure; in this case we have perfectly reversible changes of condition. 



But it is otherwise if the condensed aqueous vapor falls away from 

 the air as precipitation. The quantity of vapor remaining in the 

 air at the end of the ascension will, because of the increasing tem- 

 perature that accompanies its ascent, depart further and further 

 from its point of saturation. This process is now in descending air 

 entirely different from that which took place in ascending air, 

 wherefore von Bezold expresses the changes of condition when we 

 take into consideration the loss of the precipitation as "limited 

 reversible. " 



If we have mathematically and numerically considered the first 

 case, that of the unchanged constituents of a mass of air, then the 

 further modifications necessary for the second case, that of the 

 entire or partial loss of the precipitation, are easy to understand 

 and to apply numerically. But the latter is only possible when we 

 start, not from the ordinary assumption of thermodynamics which 

 considers a unit weight i kilogram of moist air as the basis of the 

 computation, but when we separately consider one kilogram of 

 dry air and x kilograms of aqueous vapor. Under the assumption 

 that the condensed aqueous vapor is to be assumed constant 

 then the weight (i + x) kg. of the moist air during the whole ascen- 

 sion will also be constant. The quantity x that is mixed with the 

 weight of 1 kilogram of dry air or the quantity of aqueous vapor 

 that is contained in (i + x) kg. of moist air is designated "the mix- 



